My furnace cuts off and the pilot light goes out when I light the burner and turn the heat on. I replaced the thermo coupler last year but I don't remember which one I bought

Pilot burner is probably dirty. Remove & clean. Thermocouples are pretty much universal. All I carry on the truck is 30" which will fit in 99%+ of the heaters out there. Just coil any excess. If all you can find is 24", it will likely work.

How do I replace the thermocoupler in a coleman furnace model #7680c586